The direct method of blood ammonia determination by the indophenol reaction has been adapted for use on 100 μ1 samples of whole blood. Capillary levels of ammonia nitrogen have been shown to correspond closely to arterial levels and, in contrast to venous levels, are not affected by exercise so that the method provides a simple and reliable alternative to arterial sampling. The levels of ammonia nitrogen in capillary blood from 40 adult subjects aged 18-60 years without liver disease were in the range 49-150 μg%. Mean 88 μg%. © 1968.